Method of and apparatus for eliminating crossbow in metal strip

ABSTRACT

Crossbow in metal strip in a strip processing line is eliminated immediately upstream of a strip processing device by measuring the crossbow and controlling response to the measured value penetration depth of a correcting roller into strip while the latter is supported by a pair of support rolls. The device is provided immediately upstream of the processing unit.

FIELD OF THE INVENTION

[0001] Our present invention relates to a method of eliminating crossbow in metal strip, especially steel strip, in a strip processing line and to an improved method of operating a metal processing line and to a metal processing line itself which includes a means for eliminating crossbow therein.

BACKGROUND OF THE INVENTION

[0002] In the fabrication of metal strip, a billet or slab can be rolled to the desired strip thickness in a strip processing line and the strip processing line can include other elements which can be referred to a strip processing device. These can include, in addition to the rolling mill stands, calibration stands, dressing stands, heating units for bringing the strip to an elevated temperature, for homogenizing or annealing the strip or for controlling strip temperature, cooling units for cooling the strip, descaling units for removing surface contaminants, and pickling or other units for modifying surface properties or the quality of the strip.

[0003] In the processing of the strip it is also known to eliminate or control crossbow in the strip by continuous leveling units which can effect stretch bend leveling or stretch or tension leveling of the strip. In general these devices apply tension to the strip and may subject it to bending in alternately opposite direction so as to plastically deform the strip and ultimately give rise to a high degree of straightness or planarity.

[0004] It is also known in that context, see German patent document DE 43 23 385 C1 and U.S. Pat. No. 5,535,610, to press a roller into the strip and thereby deform it while it is under tension. The degree of penetration to be controlled in accordance with the measured crossbow can serve to eliminate the crossbow downstream from a strip processing unit which is capable of producing such crossbow as the strip passes this unit. There are cases, however, in which the strip must traverse a processing device, if not in an ideally flat or planar state, only largely free from crossbow and possibly still having a certain amount of waviness or strip saber or camber. That may be the case when the strip is to be subjected to bright annealing in the processing of high quality steel strip with reduced tension in a vertical annealing furnace and in which the metal strip can loop within the furnace if it has excessive crossbow. That looping can give rise to surface damage. Waviness (corrugations) and/or strip camber is generally so small in such cases as to obviate concern for surface damage as a result of them. It is desirable however in these cases to eliminate the crossbow or to eliminate so much of the crossbow that problems do not arise without necessarily eliminating waviness and/or strip camber.

[0005] In other cases a metal strip may be subjected to a number of strip processing stages, some of which may follow the leveling action in which crossbow may be fully corrected but where such processing may contribute a degree of crossbow which may be problematic in a later processing stage.

[0006] Furthermore, crossbow correction may be required in processing lines in which there has already been some stretch bend or stretch leveling and in which a further processing device is provided. For example, downstream of the leveling unit there can be an annealing furnace followed by a cooling zone and a metal strip can leave the cooling zone with a certain amount of crossbow which may be detrimental for the subsequent strip processing, for example, pickling. In the pickling stage it is desirable that the metal strip be as free as possible from crossbow. While at first blush it might be deemed to be desirable to provide a second stretch bend and/or tension leveling unit in conjunction with the pickling stage, in practice this is impractical because it is expensive and because there is usually insufficient place for the equipment required for an additional stretch bend and/or tension leveling stage.

[0007] In another instance where crossbow has been found to be detrimental, even after a leveling operation, is a pickling line for relatively thick metal strip of, for example, carbon steel or high quality steel. In this case the metal strip may develop a crossbow after traversing the pickling stage so that the edge of the strip can cut into the routing rollers and any optional pinch rollers through which the strip is passed. The damage to the rollers may result in damage to the surface of the strip. In these cases as well, investment in stretch bend or tension leveling units may not be practical.

[0008] In all of these cases, it is desirable to minimize crossbow in metal strip upstream of a strip processing even though the strip may have been subjected to further processing on an earlier stage and even to stretch bends and/or tension leveling elsewhere along the strip processing line and further upstream of the latter strip processing device.

OBJECTS OF THE INVENTION

[0009] It is, therefore, the principal object of the present invention to provide an improved method of operating a strip processing line or method of eliminating crossbow in metal strip of a strip processing line so that drawbacks of earlier systems are avoided, the crossbow can be eliminated in a space-saving and economical manner, damage to the pinch or guide rollers of the strip processing line can be minimized and, in general, the disadvantages of earlier systems avoided.

[0010] Another object of the invention is to provide an apparatus for carrying out the method of the invention which is reliable, economical and compact.

SUMMARY OF THE INVENTION

[0011] These objects and others which will become apparent hereinafter are attained, in accordance with the invention, in a method of operating a strip processing line whereby, upstream of a strip processing device, a crossbow in the strip is detected and/for measured and, to eliminate this crossbow, immediately upstream of the strip processing device, a correcting roller is pressed into the strip as supported by a pair of support rollers on the opposite side of the strip to a depth determined by the detected degree of crossbow.

[0012] The correcting roller thereby at least partially plastically deforms the strip to eliminate the crossbow without the need for stretch bend or tension leveling devices of the type which may have been used further upstream in the processing line.

[0013] According to the invention, therefore, the crossbow is eliminated or reduced directly upstream of a strip processing unit and utilizing the detection or measurement of the crossbow, comparison of the measured or actual value of the crossbow with a setpoint value which establishes the tolerable crossbow for that particular strip processing unit, and an effector for controlling the depth of penetration of the correcting roller in response to an error signal or correction signal resulting from the comparison and sufficient to effect a semiplastic deformation of the strip to counter the crossbow.

[0014] If the strip has crossbow in only one direction, a single correcting roller suffices if it generates the semiplastic bending capable of counteracting that crossbow. However, if the metal strip has crossbow in both directions, i.e. bends outwardly to one side and the other, as for example may be the case if the metal strip is buckled or has alternating bulges and troughs across the strip, then two correcting rollers may be provided, each displaceable into the strip between respective pairs of support rolls from opposite sides of the strip.

[0015] The reference to penetration of a correcting roller into the strip to effect semiplastic deformation thereof will be understood to mean a penetration of the correcting roller into the path of the strip between two support rolls and into a plane which is tangent to both of the support rolls to a depth sufficient to effect at least partially a plastic deformation of that strip.

[0016] When the strip has a crossbow which tends to be constant, the degree of crossbow elimination can be visually determined and the depth of penetration of the correcting roller can be visually set to substantially eliminate the crossbow or reduce it to a degree which can be visually ascertained as sufficient. This setting of the correcting roller can then be stored and can determine the quality of the strip which enters the strip-treating device. One need only set the correcting roll for the particular strip quality desired to eliminate or reduce the crossbow before it enters this device to produce the stored product quality.

[0017] However, for the case in which the crossbow tends to vary, it is desirable to effect the measurement of the crossbow in a continuous or on-line basis, compare that measurement as an actual value measurement with a setpoint value for the desired depth of penetration of the correcting roller or rollers, and to regulate the depth of penetration thereof with any difference or error signal. The controller can thus be a comparator which outputs an error signal to an effector displacing the correcting roller or rollers transversely to the strip. This is especially the case when the strip has previously been subjected to stretch bend or tension leveling and has passed through a strip treatment, for example an annealing process which may effect the planarity of the strip.

[0018] The strip may be somewhat wavy or corrugated and the effect of the waviness on the measurement of the crossbow can be filtered out by integrating the measured value of the crossbow over a predetermined time period or by means of averaging to form a sliding mean. In either case, the integration or formation of a mean eliminates the effects of values which fluctuate periodically as is the case with corrugations across the width of the strip or along the edges thereof. The integration can be effected over a number of corrugation periods or maximum and minimum values can be determined to establish the mean.

[0019] When the strip is affected by strip saber or strip camber, the affect thereof on the measurements of the crossbow can be eliminated in the computer during determination of the inclined position of the crossbow curve. This inclined position as the crossbow sensor scans across the strip can be filtered out as well by determining mean values at locations spaced identically on opposite sides of the strip center, for example.

[0020] The invention also includes an apparatus for eliminating or reducing the crossbow in the metal strip which includes the correcting roller upstream of the strip processing device and the crossbow measuring system described.

BRIEF DESCRIPTION OF THE DRAWING

[0021] The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

[0022]FIG. 1 is a diagrammatic side view of a system for eliminating crossbow in metal strip according to the invention;

[0023]FIG. 2 is a diagrammatic section through a metal strip showing the crossbow and edge corrugations or waviness superimposed thereon;

[0024]FIG. 3 is a diagram showing a side view of a metal strip having the edge corrugations but without illustration of the crossbow;

[0025]FIG. 4 is a diagram showing a metal strip with pure crossbow;

[0026]FIG. 5 is a diagrammatic cross sectional view showing a metal strip with pure strip camber; and

[0027]FIG. 6 is a diagram similar to FIGS. 4 and 5 showing the combination of crossbow and strip camber.

SPECIFIC DESCRIPTION

[0028] In the drawing, we have shown a system for eliminating or reducing the crossbow in metal strip 2, e.g. steel strip, in a strip processing line. The latter should be understood to generically include any of the customary units of such a line and can include dressing stands, rolling stands, descaling units, annealing or reheating units and other strip processing units including the stretch bend levelers and/or tension levelers discussed previously. Any such equipment will be understood to be upstream of the equipment shown in FIG. 1 and FIG. 1 thus shows only a portion of the line including a further strip treating unit, e.g. a furnace or pickling line, represented at 4.

[0029] According to the invention, where the strip should not contain any crossbow upon entry into the processing device 4 or the crossbow of the strip should be limited to a certain tolerance level before entering the device and there is no room to provide for stretch bend leveling or tension leveling and/or stretch bend leveling or tension leveling is not to be used ahead of the device for some other reason, we may provide an adjustable correcting roller 5 which can be pressed into the strip to a variable depth, the strip being supported by two guide rolls 6 on the opposite side of this strip. The crossbow is here understood to be convex upwardly and thus the correcting roller 5 presses from above into the strip.

[0030] The correcting roller is vertically adjustable by an effector represented at 10, e.g. a hydraulic piston and cylinder arrangement and which receives a control signal along a line 11, e.g. in the form of hydraulic pressure.

[0031] A sensor or measuring unit 3 is provided to detect the crossbow of the strip and can scan across the latter as may be desired and provides an actual value signal along line 12 which is compared in the comparator 7 with a setpoint signal 13 representing the tolerable crossbow which can be zero. The result of that comparison is the correction, control or error signal at 11 which adjusts the correcting roller to plastically deform the strip so as to eliminate the crossbow. The unit is provided immediately upstream of the device 4 and can effectively substitute for a stretch bend or tension leveler.

[0032] If the strip has crossbow to both sides, e.g. because it bulges in part upwardly and in part downwardly, a further correcting roller 8 can be provided which can press upwardly into the strip 2 between two support rolls 9 also under the control of the comparator 7 and based upon a comparison of measured and setpoint values to eliminate the downward crossbow.

[0033] From FIGS. 2 and 3, the effect of edge waviness or corrugations 10 on the measurement will be apparent and when such waviness is present, the comparator or controller 7 can include an integrator for integrating the measured value for a predetermined time interval or for the preparation of a sliding mean value. The result is a compensation for crossbow which is not influenced by the waviness.

[0034] FIGS. 4-6 show that the strip 2 may be subjected to strip saber or camber which also can effect the measurement and which can be filtered out of the measured value of the camber by programming the computer constituting the comparator to compensate for the inclined orientation of the measured crossbow across the strip width. 

We claim:
 1. A method of eliminating crossbow in metal strip in a strip processing, comprising the steps of: (a) passing metal strip along said strip processing line through a strip processing device; (b) detecting crossbow in said strip; and (c) immediately upstream of said strip processing device, pressing a correcting roller against said strip between two support rolls to a variable depth determined by a detected degree of crossbow
 2. The method defined in claim 1 wherein respective correcting rollers are provided between respective support rolls on opposite sides of said strip for displacement of respective variable depths for eliminating crossbow in opposite directions in said strip directly before said strip enters the strip processing device.
 3. The method defined in claim 1 wherein said cross bow is eliminated downstream from a strip processing device having a negative effect on strip planarity.
 4. The method defined in claim 1 wherein in step (b) the crossbow of the strip is measured and a measured value of the crossbow is compared with a setpoint value to generate an error signal determining the depth to which said correcting roller is displaced against said strip between said support roll thereby eliminating the crossbow in said strip.
 5. The method defined in claim 1, further comprising the step of minimizing an effect of waviness in said strip in measurement of crossbow thereof by integrating measured values of said crossbow over a predetermined time period.
 6. The method defined in claim 1, further comprising the step of minimizing an effect of waviness in said strip in measurement of crossbow thereof by forming a sliding mean value filtering out the effect of waviness.
 7. The method defined in claim 1, further comprising the step of filtering out an effect of strip camber on measurement of the crossbow during a computer correction of an inclined position of the measured crossbow across the strip width.
 8. The method defined in claim 1 wherein said strip processing line includes a strip leveler and another strip processing device traversed by said strip prior to detection of crossbow and upstream of the correcting roller and the support roll.
 9. A strip processing line comprising at least one strip processing device traversed by metal strip susceptible to crossbow upstream of said strip processing device; a sensor for detecting crossbow in said strip upstream of said strip processing device; a pair of support rolls engaging said strip at one side and a correcting roller on an opposite side of said strip between said support rolls and adapted to be pressed to a variable depth against said strip between said support roll and determined by a detected degree of crossbow to at least partially deform said strip plastically and eliminate the crossbow immediately before the strip enters said strip processing device.
 10. The apparatus defined in claim 9 wherein said sensor generates a measured value signal representing the crossbow in said strip, said apparatus further comprising a circuit for comparing said measured value with a setpoint value and generating an error signal for controlling depth of penetration of said correcting roller between said support roll.
 11. The method defined in claim 10, further comprising another pair of support rolls on said opposite side of said strip and a correcting roller on said one side of said strip capable of being pressed to a variable depth against said strip whereby crossbow in said strip in opposite directions is eliminated before said strip enters said strip processing device. 